Optical pickup lens

ABSTRACT

An optical pickup lens for focusing a light beam from a laser light source on an optical information recording medium is a single lens. The optical pickup lens has two surfaces, and a surface R 2  opposite to a surface R 1  closer to the laser light source has a continuous shape. When the surface R 2  has radii h 1 , h 2  and h 3  (h 1 &lt;h 2 &lt;h 3 ) from an optical axis to a lens periphery, and where sags in the radii h 1 , h 2  and h 3  are sag 1 , sag 2  and sag 3 , and differentials in the sags are Δsag 1 , Δsag 2  and Δsag 3 , respectively, 0&gt;Δsag 1 &gt;Δsag 2  and Δsag 2 &lt;Δsag 3  are satisfied.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical pickup lens used in anoptical system for performing recording or playback of an optical disc.

2. Description of Related Art

The recording capacity of an optical disc is ever increasing, and therecording density per unit area is also increasing. When readinginformation on an optical disc, light from a light source of an opticaldisc apparatus, after passing through an optical path includingtransparent components such as a wave plate and a collimator lens, isfocused by an optical pickup lens to form an optical spot on an opticaldisc, thereby reading the information on the optical disc. Normally,light that is emitted from a laser light source enters an optical pickuplens after it is converted into parallel light by a collimator lens orthe like. For an optical pickup lens which is used for reading ahigh-capacity optical disc, laser light with a wavelength of 410 nm orshorter is used, and a numerical aperture NA is generally set to 0.84 orlarger.

An example of an optical pickup lens of a related art is disclosed inJapanese Unexamined Patent Application Publication No. 2002-156579. Theobjective lens for an optical disc which is disclosed therein is adouble-sided aspherical single lens with a numerical aperture NA of 0.7or larger, in which the center thickness of the lens is longer than afocal length. Another example is disclosed in Japanese Unexamined PatentApplication Publication No. 2001-324673. The objective lens which isdisclosed therein has an aspherical surface on one side and satisfies1.1≦d1/f≦3 where d1 is an on-axis lens thickness and f is a focallength. Yet another example is disclosed in Japanese Unexamined PatentApplication Publication No. 2002-303787. The objective lens which isdisclosed therein is a double-sided aspherical single objective lenswith a numerical aperture NA of 0.75 or larger which satisfies 1.75<nand 35<v where n is a refractive index for at least one use wavelengthand v is Abbe number on a line d.

It is necessary to maintain good off-axis characteristics in order tomount an optical pickup lens to a pickup. However, if a numericalaperture NA exceeds 0.80, it is difficult to maintain goodcharacteristics for both on-axis aberration such as spherical aberrationand off-axis aberration such as astigmatic aberration and comaticaberration of a pickup lens. Particularly, a biconvex lens in which asurface on the side close to a laser light source (surface R1) and asurface on the opposite side (surface R2) are both convex is difficultto have good angle of view characteristics.

Further, if a numerical aperture NA exceeds 0.80, a working distance(WD) which indicates a distance between an optical pickup lens and anoptical disc becomes smaller. The decrease in the working distance isparticularly significant in a meniscus lens in which the surface R2 isnot convex, thus increasing the risk of collision between an opticaldisc and an optical pickup lens.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above problemsand an object of the present invention is thus to provide an opticalpickup lens with a high NA which maintains a longer working distance andhas good on-axis characteristic and off-axis characteristics.

According to one aspect of the present invention, there is provided anoptical pickup lens for focusing a light beam from a laser light sourceon an optical information recording medium. The optical pickup lens is asingle lens, and a second surface of the optical pickup lens opposite toa first surface closer to the laser light source has a continuous shape.Further, a shape of the second surface of the optical pickup lens havingradii h1, h2 and h3 (h1<h2<h3) from an optical axis to a lens periphery,where sags in the radii h1, h2 and h3 are sag1, sag2 and sag3, anddifferentials in the sags are Δsag1, Δsag2 and Δsag3, respectively,satisfies 0>Δsag1>Δsag2 and Δsag2<Δsag3.

Because the shape of the second surface satisfies 0>Δsag1>Δsag2 andΔsag2<Δsag3, it is possible to maintain a sufficient working distance (adistance between an optical pickup lens and an optical disc) and obtaingood on-axis aberration characteristics.

The shape of the second surface of the above optical pickup lenspreferably has radii h1, h2, h3 and h4 which satisfy sag2>sag3 andsag3<sag4 where a sag in the radius h4 (h3<h4) is a sag 4.Alternatively, the shape of the second surface of the above opticalpickup lens preferably has radii h1, h2, h3 and h4 which satisfy Δsag3<0and Δsag4>0 where a sag in a lens radius h4 (h3<h4) is a sag 4 and adifferential in the sag is Δsag4. Also in the above optical pickup lens,the shape of the second surface preferably has a minimum. It is alsopreferred in the above optical pickup lens that the second surface isconvex in a center and concave in a periphery. By satisfying suchconditions, it is possible to maintain a longer working distance andobtain better on-axis characteristics and off-axis characteristics.

It is further preferred in the above optical pickup lens that 0.84≦NAand 0.9≦d/f are satisfied when the optical pickup lens is used in arecording and/or playback pickup apparatus using a laser with awavelength of 410 nm or shorter and where a numerical aperture is NA, asingle lens center thickness is d, and a focal length is f.

It is more preferred in the above optical pickup lens that 0.84≦NA and0.9≦d/f≦1.2 are satisfied when the optical pickup lens is used in arecording and/or playback pickup apparatus using a laser with awavelength of 410 nm or shorter and where a numerical aperture is NA, asingle lens center thickness is d, and a focal length is f. Bysatisfying such conditions, it is possible to maintain a suitable edgethickness and facilitate the manufacture of the shape of the secondsurface which satisfies 0>Δsag1>Δsag2 and Δsag2<Δsag3.

Further, in the above optical pickup lens, a refractive index n for awavelength of 405 nm is preferably 1.51≦n≦1.64.

More preferably, a refractive index n for a wavelength of 405 nm is1.59≦n≦1.62. By satisfying such conditions, it is possible to maintain asuitable edge thickness and facilitate the design of the second surfacewhich satisfies 0>Δsag1>Δsag2 and Δsag2<Δsag3.

Furthermore, in the above optical pickup lens, an effective diameter Dof the single lens is preferably 1.8≦D≦3.2 mm. By applying the presentinvention to the lens having such an effective diameter, it is possibleto maintain a long working distance and improve on-axis characteristicsand off-axis characteristics.

In addition, aberration is preferably 15 mλrms when an angle of view asoff-axis characteristics is 0.3 degrees.

It is also preferred in the above optical pickup lens that a tangentangle α of the first surface is 60°≦α. If the tangent angle α becomeslarger, the sag in the first surface increases, and the sag in thesecond surface decreases accordingly. This facilitates the manufactureof the shape of the second surface which satisfies 0>Δsag1>Δsag2 andΔsag2<Δsag3.

It is further preferred that an Abbe number vd of a lens material is50≦vd. This enables accurate writing of a pit sequence on an opticaldisc. As the Abbe number is larger, the lens is more resistant to thedisplacement of a wavelength during writing.

Further, parallel light or sub-finite light may enter the optical pickuplens through the first surface. The single lens may be made of plasticor glass.

The present invention provides an optical pickup lens with a high NAwhich maintains a longer working distance and has good on-axischaracteristic and good off-axis characteristics.

The above and other objects, features and advantages of the presentinvention will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing an optical pickup lens according to anembodiment of the present invention;

FIG. 1B is a schematic view showing a part within a dotted line in FIG.1A in an enlarged scale;

FIG. 2A is a view showing another optical pickup lens according to anembodiment of the present invention;

FIG. 2B is a schematic view showing a part within a dotted line in FIG.2A in an enlarged scale;

FIG. 3 is a view to describe a lens center thickness d, an effectivediameter D and a working distance WD of an optical pickup lens accordingto an embodiment of the present invention;

FIG. 4 is a view showing wavefront aberration in examples of the presentinvention;

FIGS. 5A to 5D are views showing characteristic values of an opticalpickup lens according to an example 1 of the present invention;

FIG. 6A is a view showing longitudinal aberration;

FIG. 6B is a view showing a sag in the radial direction;

FIG. 6C is a view showing an optical pickup lens according to theexample 1;

FIGS. 7A to 7D are views showing characteristic values of an opticalpickup lens according to an example 2 of the present invention;

FIG. 8A is a view showing longitudinal aberration;

FIG. 8B is a view showing a sag in the radial direction;

FIG. 8C is a view showing an optical pickup lens according to theexample 2;

FIGS. 9A to 9D are views showing characteristic values of an opticalpickup lens according to an example 3 of the present invention;

FIG. 10A is a view showing longitudinal aberration;

FIG. 10B is a view showing a sag in the radial direction;

FIG. 10C is a view showing an optical pickup lens according to theexample 3;

FIGS. 11A to 11D are views showing characteristic values of an opticalpickup lens according to an example 4 of the present invention;

FIG. 12A is a view showing longitudinal aberration;

FIG. 12B is a view showing a sag in the radial direction;

FIG. 12C is a view showing an optical pickup lens according to theexample 4;

FIGS. 13A to 13D are views showing characteristic values of an opticalpickup lens according to an example 5 of the present invention;

FIG. 14A is a view showing longitudinal aberration;

FIG. 14B is a view showing a sag in the radial direction;

FIG. 14C is a view showing an optical pickup lens according to theexample 5;

FIGS. 15A to 15D are views showing characteristic values of an opticalpickup lens according to an example 6 of the present invention;

FIG. 16A is a view showing longitudinal aberration;

FIG. 16B is a view showing a sag in the radial direction;

FIG. 16C is a view showing an optical pickup lens according to theexample 6;

FIGS. 17A to 17D are views showing characteristic values of an opticalpickup lens according to an example 7 of the present invention;

FIG. 18A is a view showing longitudinal aberration;

FIG. 18B is a view showing a sag in the radial direction;

FIG. 18C is a view showing an optical pickup lens according to theexample 7;

FIGS. 19A to 19D are views showing characteristic values of an opticalpickup lens according to an example 8 of the present invention;

FIG. 20A is a view showing longitudinal aberration;

FIG. 20B is a view showing a sag in the radial direction;

FIG. 20C is a view showing an optical pickup lens according to theexample 8;

FIGS. 21A to 21D are views showing characteristic values of an opticalpickup lens according to an example 9 of the present invention;

FIG. 22A is a view showing longitudinal aberration;

FIG. 22B is a view showing a sag in the radial direction;

FIG. 22C is a view showing an optical pickup lens according to theexample 9;

FIGS. 23A to 23D are views showing characteristic values of an opticalpickup lens according to an example 10 of the present invention;

FIG. 24A is a view showing longitudinal aberration;

FIG. 24B is a view showing a sag in the radial direction;

FIG. 24C is a view showing an optical pickup lens according to theexample 10;

FIGS. 25A to 25D are views showing characteristic values of an opticalpickup lens according to an example 11 of the present invention;

FIG. 26A is a view showing longitudinal aberration;

FIG. 26B is a view showing a sag in the radial direction;

FIG. 26C is a view showing an optical pickup lens according to theexample 11;

FIGS. 27A to 27D are views showing characteristic values of an opticalpickup lens according to an example 12 of the present invention;

FIG. 28A is a view showing longitudinal aberration;

FIG. 28B is a view showing a sag in the radial direction;

FIG. 28C is a view showing an optical pickup lens according to theexample 12;

FIGS. 29A to 29D are views showing characteristic values of an opticalpickup lens according to an example 13 of the present invention;

FIG. 30A is a view showing longitudinal aberration;

FIG. 30B is a view showing a sag in the radial direction;

FIG. 30C is a view showing an optical pickup lens according to theexample 13;

FIGS. 31A to 31D are views showing characteristic values of an opticalpickup lens according to an example 14 of the present invention;

FIG. 32A is a view showing longitudinal aberration;

FIG. 32B is a view showing a sag in the radial direction;

FIG. 32C is a view showing an optical pickup lens according to theexample 14;

FIGS. 33A to 33D are views showing characteristic values of an opticalpickup lens according to an example 15 of the present invention;

FIG. 34A is a view showing longitudinal aberration;

FIG. 34B is a view showing a sag in the radial direction;

FIG. 34C is a view showing an optical pickup lens according to theexample 15;

FIGS. 35A to 35D are views showing characteristic values of an opticalpickup lens according to an example 16 of the present invention;

FIG. 36A is a view showing longitudinal aberration;

FIG. 36B is a view showing a sag in the radial direction;

FIG. 36C is a view showing an optical pickup lens according to theexample 16;

FIGS. 37A to 37D are views showing characteristic values of an opticalpickup lens according to an example 17 of the present invention;

FIG. 38A is a view showing longitudinal aberration;

FIG. 38B is a view showing a sag in the radial direction;

FIG. 38C is a view showing an optical pickup lens according to theexample 17;

FIGS. 39A to 39D are views showing characteristic values of an opticalpickup lens according to an example 18 of the present invention;

FIG. 40A is a view showing longitudinal aberration;

FIG. 40B is a view showing a sag in the radial direction;

FIG. 40C is a view showing an optical pickup lens according to theexample 18;

FIGS. 41A to 41D are views showing characteristic values of an opticalpickup lens according to an example 19 of the present invention;

FIG. 42A is a view showing longitudinal aberration;

FIG. 42B is a view showing a sag in the radial direction;

FIG. 42C is a view showing an optical pickup lens according to theexample 19;

FIGS. 43A to 43D are views showing characteristic values of an opticalpickup lens according to an example 20 of the present invention;

FIG. 44A is a view showing longitudinal aberration;

FIG. 44B is a view showing a sag in the radial direction;

FIG. 44C is a view showing an optical pickup lens according to theexample 20;

FIGS. 45A to 45D are views showing characteristic values of an opticalpickup lens according to an example 21 of the present invention;

FIG. 46A is a view showing longitudinal aberration;

FIG. 46B is a view showing a sag in the radial direction;

FIG. 46C is a view showing an optical pickup lens according to theexample 21;

FIGS. 47A to 47D are views showing characteristic values of an opticalpickup lens according to an example 22 of the present invention;

FIG. 48A is a view showing longitudinal aberration;

FIG. 48B is a view showing a sag in the radial direction;

FIG. 48C is a view showing an optical pickup lens according to theexample 22;

FIGS. 49A to 49D are views showing characteristic values of an opticalpickup lens according to an example 23 of the present invention;

FIG. 50A is a view showing longitudinal aberration;

FIG. 50B is a view showing a sag in the radial direction;

FIG. 50C is a view showing an optical pickup lens according to theexample 23;

FIGS. 51A to 51D are views showing characteristic values of an opticalpickup lens according to an example 24 of the present invention;

FIG. 52A is a view showing longitudinal aberration;

FIG. 52B is a view showing a sag in the radial direction;

FIG. 52C is a view showing an optical pickup lens according to theexample 24;

FIGS. 53A to 53D are views showing characteristic values of an opticalpickup lens according to an example 25 of the present invention;

FIG. 54A is a view showing longitudinal aberration;

FIG. 54B is a view showing a sag in the radial direction;

FIG. 54C is a view showing an optical pickup lens according to theexample 25;

FIGS. 55A to 55D are views showing characteristic values of an opticalpickup lens according to an example 26 of the present invention;

FIG. 56A is a view showing longitudinal aberration;

FIG. 56B is a view showing a sag in the radial direction;

FIG. 56C is a view showing an optical pickup lens according to theexample 26;

FIGS. 57A to 57D are views showing characteristic values of an opticalpickup lens according to an example 27 of the present invention;

FIG. 58A is a view showing longitudinal aberration;

FIG. 58B is a view showing a sag in the radial direction;

FIG. 58C is a view showing an optical pickup lens according to theexample 27;

FIGS. 59A to 59D are views showing characteristic values of an opticalpickup lens according to an example 28 of the present invention;

FIG. 60A is a view showing longitudinal aberration;

FIG. 60B is a view showing a sag in the radial direction;

FIG. 60C is a view showing an optical pickup lens according to theexample 28;

FIGS. 61A to 61D are views showing characteristic values of an opticalpickup lens according to an example 29 of the present invention;

FIG. 62A is a view showing longitudinal aberration;

FIG. 62B is a view showing a sag in the radial direction;

FIG. 62C is a view showing an optical pickup lens according to theexample 29;

FIGS. 63A to 63D are views showing characteristic values of an opticalpickup lens according to an example 30 of the present invention;

FIG. 64A is a view showing longitudinal aberration;

FIG. 64B is a view showing a sag in the radial direction;

FIG. 64C is a view showing an optical pickup lens according to theexample 30;

FIGS. 65A to 65D are views showing characteristic values of an opticalpickup lens according to an example 31 of the present invention;

FIG. 66A is a view showing longitudinal aberration;

FIG. 66B is a view showing a sag in the radial direction;

FIG. 66C is a view showing an optical pickup lens according to theexample 31;

FIGS. 67A to 67D are views showing characteristic values of an opticalpickup lens according to an example 32 of the present invention;

FIG. 68A is a view showing longitudinal aberration;

FIG. 68B is a view showing a sag in the radial direction; and

FIG. 68C is a view showing an optical pickup lens according to theexample 32.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is described hereinafter indetail with reference to the drawings. In the following embodiment, thepresent invention is applied to an optical pickup lens used forrecording or playing back information on an optical informationrecording medium.

FIG. 1A is a view showing an optical pickup lens according to theembodiment of the present invention. FIG. 1B is a view to describe thesag of the optical pickup lens, which schematically shows a part withina dotted line in FIG. 1A in an enlarged scale. As shown in FIG. 1A, anoptical pickup lens 1 a, which is a single lens, of this embodiment hasa first surface (which is referred to hereinafter as a surface R1) 11that is closer to a laser light source and a second surface (hereinafteras a surface R2) 12 a that is opposite to the first surface 11 and thatfaces an optical disc 30 which includes an optical disc substrate 32 anda light transmitting layer 31 in an optical disc. The surface R2 12 ahas the following shape.

If the lens surface has radius h1<radius h2<radius h3 from the center tothe periphery of the lens, and differentials in sag in the radius h1,the radius h2 and the radius h3 are Δsag1, Δsag2 and Δsag3,respectively, the shape of the surface R2 12 a satisfies the followingexpressions (1) and (2):0>Δsag1>Δsag2  (1)Δsag2<Δsag3  (2)

Firstly, a sag and a differential in sag (Δsag) are describedhereinafter. FIG. 1B schematically shows the surface R2 12 a from acenter h0 to a periphery 13 a. As shown in FIG. 1B, when the opticalpickup lens 1 a is disposed in such a way that the optical axis and thelens center h0 of the surface R2 are in the corresponding position, thesag is a distance from the normal L to the optical axis on the lenscenter h0 to the surface R2 in a given radius h. The direction from thesurface R1 11 to the surface R2 12 a is referred to as a positive. Adifferential in sag (Δsag) is the slope of the sag in a given radius hon the surface R2 12 a, which is, the slope of the tangent to thesurface R2 from the line L in a radius h. An increase in sag from theinside to the outside is referred to as a positive, and a decrease isreferred to as a negative.

In FIGS. 1A and 1B, when the sags of the radii h1, h2 and h3 are sag1,sag2 and sag3, respectively, the following relationships are satisfied:h1<h2<h3, andsag1>sag2>sag3

FIG. 2A shows another optical pickup lens according to the embodiment ofthe present invention. FIG. 2B schematically shows a part within adotted line in FIG. 2A in an enlarged scale. The optical pickup lens mayhave the shape shown in FIGS. 2A and 2B rather than the shape shown inFIGS. 1A and 1B described above. Specifically, as shown in FIGS. 2A and2B, a surface R2 12 b has the shape which satisfies the above-describedexpressions (1) and (2) and further has a minimum k (radius hk).

Having the minimum means that, when the lens has radii h1<h2<h3<h4 withsag1, sag2, sag3 and sag4, respectively, the following relationships aresatisfied:sag1>sag2>sag3  (3)sag3<sag4  (4)Specifically, the surface R2 12 b has the radii h1 to h4 which satisfythe expressions (3) and (4).

Further, if the lens radii h1, h2, h3 and h4 satisfy h1<h2<h3<h4, anddifferentials in sag in the radii h1, h2, h3 and h4 are Δsag1, Δsag2,Δsag3 and Δsag4, respectively, the shape of the surface R2 12 bsatisfies the following expression (5) as well as the expressions (1)and (2):0>Δsag1>Δsag2  (1)Δsag2<Δsag3<0  (2)Δsag4>0  (5)

In FIGS. 1A and 1B, the lens surface is convex-shaped in its centerpart, so that the slope (Δsag) is first gradual, then abrupt, andgradual again from the lens center h0 to the outside. In FIGS. 2A and2B, the lens surface further has a concave-shaped portion in itsperiphery, so that concave and convex shapes are alternately arranged inthe radial direction. Specifically, the surface R2 12 b of the opticalpickup lens 1 b shown in FIGS. 2A and 2B has the shape that the centerpart is convex and the peripheral part is concave.

The surface R2 of the optical pickup lenses shown in FIGS. 1A to 2B hasa continuous shape, and there is no ring-zone structure with steps suchas a diffraction lens. However, two or three steps may be formed withoutdeparting from the technical gist of the present invention. The surfaceR1 may have a continuous shape or a ring-zone structure with steps.

The surface R2 of the optical pickup lens is good enough if it has theshape with a continuous slope, which includes the shape that the surfaceR2 is continuous and the shape that the continuous shape of the surfaceR2 is shifted in parallel to the optical axis direction in a part of thelens surface.

With the surfaces R2 12 a and 12 b having such a shape, the followingadvantages are obtained. A general pickup lens is a biconvex lens whichhas convex surfaces on both sides or a meniscus lens which is composedof a convex lens and a concave lens. Each lens has different features.Specifically, a working distance (WD), which is a distance from thecenter of the surface R2 to the optical disc 30, is shorter in themeniscus lens than in the biconvex lens because the surface R2 of themeniscus lens is concave. The angle of view characteristics, which arethe off-axis characteristics, are better in the meniscus lens than inthe biconvex lens because the both surfaces of the meniscus lens arecurved in the same direction.

On the other hand, since the optical pickup lenses 1 a and 1 b of thisembodiment have a concave portion in the periphery and a convex portionin the center, they satisfies the both features of the meniscus lens andthe biconvex lens. Specifically, the optical pickup lenses 1 a and 1 bmaintain a longer working distance and achieves good angle of viewcharacteristics as off-axis characteristics while maintaining goodon-axis characteristics.

In the optical pickup lens 1 a shown in FIGS. 1A and 1B, the sagincreases gradually from the center h0 toward the periphery 13 a alongthe radius of the surface R2 12 a and, after reaching a certain radius,which is the radius h3 in the example of FIG. 1B, the sag does notsubstantially change. In the optical pickup lens 1 b shown in FIGS. 2Aand 2B, the sag increases gradually from the center h0 toward theperiphery 13 b along the radius of the surface R2 12 b and, afterreaching a certain radius, which is the radius h4 in the example of FIG.2B, the sag decreases gradually to the periphery 13 b. The part of thelens which is inside of the position where an increase or decreaseamount of sag differentials has the characteristics of a biconvex lens,and the part of the lens which is outside of that position has thecharacteristics of a meniscus lens.

Specifically, because the center part of the lens serves as a biconvexlens, a working distance is long, and since a curvature radius is notvery large, the angle of view characteristics are good in spite of beinga biconvex lens. Further, because the part of the lens which is outsideof the position where an increase or decrease amount of a sagdifferentials, which is the peripheral part of the surface R2, has thefeature of a meniscus lens, good angle of view characteristics, whichare the advantage of the meniscus lens, can be obtained. Furthermore,because the portion which corresponds to a meniscus lens is formed notin the center part of the lens but in the peripheral part of the lens, aworking distance is not shortened. In this manner, the optical pickuplenses 1 a and 1 b of this embodiment have a substantially flat orconcave portion in the outer part to exert the characteristics of ameniscus lens and have a convex portion in the inner part to exert thecharacteristics of a biconvex lens, thereby maintaining a long workingdistance and achieving good angle of view characteristics as well asgood on-axis characteristics. The radii h1 to h4 in FIGS. 1B and 2B arepreferably located in the region through which a laser light beampasses.

Although the optical pickup lens 1 a has a meniscus shape in the partoutside of the position of the radius h3 where a sag increase/decreaseamount changes as shown in FIGS. 1A and 1B, the optical pickup lens 1 bhas an extreme meniscus shape with a minimum k as shown in FIGS. 2A and2B. Such a shape enables a longer working distance and better angle ofview characteristics as off-axis characteristic and on-axischaracteristics.

When the optical pickup lenses 1 a and 1 b are used in a recordingand/or playback pickup apparatus which uses a laser with a wavelength of410 nm or shorter as a light source used in an optical head or anoptical disc apparatus, it is preferred to satisfy the followingexpressions:0.84≦NA0.9≦d/f≦1.2

where NA is a numerical aperture of the optical pickup lens, d is asingle lens center thickness of the optical pickup lens (cf. FIG. 3),and f is s focal length.

If the numerical aperture NA is smaller than 0.84, the effectivediameter of the surface R2 becomes smaller. If the effective diameter ofthe surface R2 becomes smaller, it becomes difficult to form theboundary between a biconvex lens portion and a meniscus lens portion inthe peripheral part of the surface R2 as described above. Therefore, thenumerical aperture NA is preferably 0.8 or larger and more preferably0.84 or larger.

In order to increase the working distance, it is generally preferred toreduce the center thickness d and lower the refractive index n. On theother hand, in order to set the numerical aperture NA to be 0.84 orlarger, increase the working distance and improve the angle of viewcharacteristics, it is preferred to regulate the lens performance, whichis the relationship d/f of the focal length f and the center thicknessd.

The reason that d/f of 0.9 or larger is preferred is as follows. If thefocal length f is a fixed value, the value of d/f decreases as thecenter thickness d becomes smaller. The decrease in d/f results in thereduction of a distance between the surface R1 and the surface R2 in theperipheral edge, which is called an edge thickness. The reduction of theedge thickness causes a problem such as edge cracking, which makes itdifficult to mount a lens. Further, because f=h/NA where f is a focallength and h is a radius, the relationship of d/f=d*NA/h is satisfied.If the numerical aperture NA is a fixed value since it is preferably0.84 or larger, the value of d/f decreases as the radius is set larger,and it is thus needed to increase the center thickness d accordingly soas to maintain a sufficient edge thickness. It is therefore preferred toset the value of d/f to be 0.9 or larger.

If the value of d/f is 1.2 or smaller, the shape of the surface R2 canbe formed easily. It is therefore preferred to set the value of d/f tobe 1.2 or smaller.

Setting the numerical aperture NA to be 0.84 or larger and the value ofd/f to be 0.9 or larger facilitates the design of the optical pickuplens having the shape shown in FIGS. 1A to 2B, thus obtaining a longworking distance and good angle of view characteristics as well as goodon-axis characteristics.

The refractive index is preferably set to 1.51≦n≦1.64 where n is arefractive index for a blue-violet laser with a wavelength of 405 nm. Ifthe refractive index n is smaller than 1.51, a curvature is largercompared with a lens having the same center thickness and a largerrefractive index, which causes the reduction of the edge thickness,which is a distance between the peripheral edges of the surface R1 andthe surface R2. It is therefore preferred that the refractive index is1.51 or larger.

On the other hand, if the refractive index n is larger than 1.64, it isdifficult to maintain the shape of the surface R2 of the presentinvention which has both the features of a biconvex lens and a meniscuslens, and it becomes closer to a regular meniscus lens. It is thereforepreferred that the refractive index is 1.64 or smaller. However, whenthe center part of the surface R2 is convex and the peripheral part isconcave, the refractive index n can be larger than 1.64.

It is further preferred that the refractive index is set to 1.59≦n≦1.62.Setting the refractive index n to the range of 1.59 to 1.62 facilitatesthe design of the optical pickup lens having the shape shown in FIGS. 2Aand 2B. It is therefore preferred to set the refractive index to therange of 1.59 to 1.62.

Setting the refractive index n to the range of 1.51 to 1.64 or the rangeof 1.59 to 1.62 makes it possible to design the optical pickup lenshaving the shapes shown in FIGS. 1A to 2B easily. It is thereby possibleto maintain a long working distance and improve the angle of viewcharacteristics as well as the on-axis characteristics. The opticalpickup lens has a practical lens diameter.

It is also preferred that the effective diameter D (cf. FIG. 3) is setto 1.8≦D≦3.2 mm. If the effective diameter D is larger than 3.2 mm, aworking distance is too long and it is difficult to manufacture thelens. If, on the other hand, the effective diameter D is smaller than1.8 mm, a working distance is too short and it is not practical. It istherefore preferred that the effective diameter D is in the range of 1.8mm to 3.2 mm.

It is also preferred that the tangent angle α of the surface R1, whichis one of the two surfaces of a single lens (optical pickup lens) thatis closer to a laser light source, is 60°≦α. As the tangent angle αbecomes larger, the sag of the surface R1 increases and the sag of thesurface R2 decreases accordingly, which makes it easier to manufacturethe shape of the optical pickup lens shown in FIGS. 1A, 1B and 2A, 2B.On the other hand, if the tangent angle α is smaller than 60 degrees,the sag of the surface R1 decreases and the sag of the surface R2increases accordingly. This makes it difficult to manufacture the shapeof the surface R2 and deteriorates the angle of view characteristics. Itis therefore preferred that the tangent angle α of the surface R1 is 60°or larger. It is thereby possible to facilitate the manufacture of theshape of the surface R2 and obtain good angle of view characteristics aswell as good on-axis characteristics.

It is further preferred that the Abbe number vd is 50≦vd. As a lens hasa larger Abbe number, chromatic aberration improves in the pickup lens.The chromatic aberration indicates a displacement of a best spotposition when a wavelength is deviated by +1 nm each. In a pickup lens,a laser power is turned up when performing recording. The increase inthe laser power causes a temporal deviation of a wavelength to belonger. If a best spot position is displaced during recording, trackingcan be deviated and recording on the best spot position becomesdifficult. It is thus necessary to increase the Abbe number in order tomaintain good recording characteristics. The Abbe number is inverselyproportional to the refractive index. The refractive index is preferably1.51≦n≦1.64 as described earlier, and with such a refractive index, theAbbe number is about 50≦vd≦81. Therefore, the Abbe number is preferably50 or larger, and more preferably 60 or larger.

Examples of the present invention are described hereinafter. FIG. 4shows wavefront aberration in examples 1 to 32. The details of theexample 1 are shown in FIGS. 5A to 5D and FIGS. 6A to 6C, the details ofthe example 2 are shown in FIGS. 7A to 7D and FIGS. 8A to 8C, and thedetails of the example 3 are shown in FIGS. 9A to 9D and FIGS. 10A to10C. Likewise, the details of the examples in FIG. 4 are respectivelyshown in the subsequent figures up to the example 32 shown in FIGS. 67Ato 67D and FIGS. 68A to 68C. Further, the examples 1 to 4 correspond tothe optical pickup lens 1 a shown in FIGS. 1A and 1B. The examples 5 to32 correspond to the optical pickup lens 1 b shown in FIGS. 2A and 2B.Referring to the example 1, for instance, the corresponding FIGS. 5A to5D show the characteristic values of the optical pickup lens. FIG. 6Ashows longitudinal aberration, FIG. 6B shows the sag from the center tothe periphery of the surface R2, and FIG. 6C illustrates the opticalpickup lens of the example 1.

The coefficients in the examples 1 to 32 are described hereinafter. Theexpression Z₁(h₁) indicating the curve of the surface R1 of the opticalpickup lens is represented by the following expression (6):

$\begin{matrix}{{Z_{1}\left( h_{1} \right)} = {\frac{h_{1}^{1}}{R_{1}\left( {1 + \sqrt{1 - \frac{\left( {1 + k_{1}} \right)h_{1}^{2}}{R_{1}^{2}}}} \right)} + {A_{1}4\; h_{1}^{4}} + {A_{1}6\; h_{1}^{6}} + {A_{1}8\; h_{1}^{8}} + {A_{1}10\; h_{1}^{10}} + {A_{1}12\; h_{1}^{12}} + {A_{1}14\; h_{1}^{14}} + {A_{1}16\; h_{1}^{16}} + \ldots}} & (6)\end{matrix}$

where

Z₁(h₁) is the sag of the surface R1 of the optical pickup lens at theheight h₁ from the optical axis,

h₁ is the height from the optical axis,

k₁ is the constant of the cone of the surface R1 of the optical pickuplens,

A1 ₄, A1 ₆, A1 ₈, A1 ₁₀, A1 ₁₂, A1 ₁₄ and A1 ₁₆ are asphericcoefficients of the surface R1 of the optical pickup lens, and

R₁ is the curvature radius of the surface R1 of the optical pickup lens.

The expression Z₂(h₂) indicating the curve of the surface R2 of theoptical pickup lens is represented by the following expression (7):

$\begin{matrix}{{Z_{2}\left( h_{2} \right)} = {\frac{h_{2}^{2}}{R_{2}\left( {1 + \sqrt{1 - \frac{\left( {1 + k_{2}} \right)h_{2}^{2}}{R_{2}^{2}}}} \right)} + {A_{2}4\; h_{2}^{4}} + {A_{2}6\; h_{2}^{6}} + {A_{2}8\; h_{2}^{8}} + {A_{2}10\; h_{2}^{10}} + {A_{2}12\; h_{2}^{12}} + {A_{2}14\; h_{2}^{14}} + {A_{2}16\; h_{2}^{16}} + \ldots}} & (7)\end{matrix}$

where

Z₂(h₂) is the sag of the surface R2 of the optical pickup lens at theheight h₂ from the optical axis,

h₂ is the height from the optical axis,

k₂ is the constant of the cone of the surface R2 of the optical pickuplens,

A2 ₄, A2 ₆, A2 ₈, A2 ₁₀, A2 ₁₂, A2 ₁₄ and A2 ₁₆ are asphericcoefficients of the surface R2 of the optical pickup lens, and

R₂ is the curvature radius of the surface R2 of the optical pickup lens.

The example 1 shown in FIGS. 5A to 5D and FIGS. 6A to 6C as a typicalexample of the optical pickup lens 1 a shown in FIGS. 1A and 1B, and theexample 5 shown in FIGS. 13A to 13D and FIGS. 14A to 14C as a typicalexample of the optical pickup lens 1 b shown in FIGS. 2A and 2B aredescribed hereinbelow. The single lens (optical pickup lens) of theexample 1 has a curvature radius R, an inter-surface distance d, arefractive index n for a wavelength 504 nm, and an Abbe number vd asshown in FIG. 5A. FIG. 6A shows the result of measuring longitudinalaberration for the single lens (optical pickup lens) of the example 1and an optical disc. FIG. 5B shows the focal length d, the workingdistance WD, the numerical aperture NA and the effective diameter of theoptical pickup lens of the example 1, FIGS. 5C and 5D show the asphericcoefficients of the surfaces R1 and R2, respectively, and FIG. 6C showsa schematic illustration of the optical pickup lens. FIG. 6B shows theshape of the surface R2 as a graph with the vertical axis indicatingfrom the center h0 to the peripheral edge (effective diameter) and thehorizontal axis indicating the sag. As shown in FIG. 6B, the example 1satisfies:0>Δsag1>Δsag2  (1)Δsag2<Δsag3  (2)andh1<h2<h3, andsag1>sag2>sag3in the range of the lens effective diameter just like the optical pickuplens 1 a shown in FIGS. 1A and 1B.

Now, this point is described in detail based on the data shown in FIG.5. If it is assumed that h1=0.100 mm, h2=0.500 mm, and h3=0.950 mm inthe example 1, it is obtained that sag1=−0.00365 mm, sag2=−0.04780 mm,and sag3=0.07172 mm. Next, sags when 0.005 mm is added to each radiush1, h2, and h3 are calculated to obtain Δsag1, Δsag2, and Δsag3.sag in h1+0.005 mm=0.105 mm equals to −0.00400 mm,sag in h2+0.005 mm=0.505 mm equals to −0.04831 mm, andsag in h3+0.005 mm=0.955 mm equals to −0.07176 mm.Therefore,Δsag1=((−0.00400)−(−0.00305))/(0.105−0.100)=−0.0704,Δsag2=((−0.04831)−(−0.04780))/(0.505−0.500)=−0.1020,andΔsag3=((−0.07176)−(−0.07172))/(0.955−0.950)=−0.0080.

As can be shown in the verification above based on the specificnumerical values, the example 1 satisfies the above-described expressionin the range of the lens effective diameter just like the optical pickuplens 1 a shown in FIGS. 1A and 1B.

The optical pickup lens of the example 1 with the surface R2 of such ashape has suitable longitudinal aberration within the lens effectivediameter as shown in FIG. 6A. It also has good angle of viewcharacteristics as shown in FIG. 4.

As shown in FIG. 14B, the example 5 satisfies:sag1>sag2>sag3  (3)sag3<sag4  (4)and0>Δsag1>Δsag2  (1)Δsag2<Δsag3<0  (2)Δsag4>0  (5)just like the optical pickup lens 1 b shown in FIGS. 2A and 2B.

Now, this point is described in detail based on the data shown in FIG.13. If it is assumed that h1=0.100 mm, h2=0.600 mm, h3=0.700 mm, andh4=1.200 mm in the example 5, it is obtained that sag1=−0.00141 mm,sag2=−0.02460 mm, sag3=−0.02690 mm, and sag4=−0.01280 mm. Next, sagswhen 0.005 mm is added to each radius h1, h2, h3, and h4 are calculatedto obtain Δsag1, Δsag2, Δsag3 and Δsag4.sag in h1+0.005 mm=0.105 mm equals to −0.00155 mm,sag in h2+0.005 mm=0.605 mm equals to −0.02440 mm,sag in h3+0.005 mm=0.705 mm equals to −0.02698 mm, andsag in h4+0.005 mm=1.205 mm equals to −0.01246 mm.Therefore,Δsag1=((−0.00155)−(−0.00141))/(0.105−0.100)=−0.0280,Δsag2=((−0.02440)−(−0.02460))/(0.605−0.600)=−0.0400,Δsag3=((−0.02698)−(−0.02690))/(0.705−0.700)=−0.0160,andΔsag4=((−0.01246)−(−0.01280))/(1.205−1.200)=+0.068.

As can be shown in the verification above based on the specificnumerical values, the example 1 satisfies the above-described expressionin the range of the lens effective diameter just like the optical pickuplens 1 b shown in FIGS. 2A and 2B.

The optical pickup lens of the example 5 with the surface R2 of such ashape has suitable longitudinal aberration within the lens effectivediameter as shown in FIG. 14A. It also has good angle of viewcharacteristics as shown in FIG. 4.

Likewise, the optical pickup lenses of the other examples have suitablelongitudinal aberration within the lens effective diameter and also havegood angle of view characteristics as shown in FIG. 4.

Therefore, it is possible in the examples 1 to 32 shown in FIGS. 4 and5A to 68C to manufacture the optical pickup lenses with a high NA whichmaintain a longer working distance and maintains suitable sineconditions and good off-axis characteristics.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. An optical pickup lens with a numerical aperture of 0.84 or largerfor focusing a light beam of a collimated light or a weak finite lightin a coherent wave-front state from a laser light source with awavelength of 410 nm or shorter on an optical information recordingmedium in such a state that, as off-axis characteristics, an aberrationbecomes 15 mλrms or smaller when the angle of view is 0.3 degrees, theoptical pickup lens being a biconvex single lens with a first surfacefrom which the light beam from the laser light source enters withoutpassing through any diffraction ring zone and a second surface oppositeto the first surface, the first surface and the second surface havingconvex shapes, wherein the second surface has a continuous shape, ashape of the second surface of the optical pickup lens having radii h1,h2 and h3 from an optical axis to a lens periphery, the radii satisfyinga first expression h1<h2<h3, wherein sags in the radii h1, h2, and h3are sag1, sag2, and sag3, and differentials in the sags are Δsag1,Δsag2, and Δsag3, respectively, the differentials satisfying second andthird expressions 0>Δsag1>Δsag2 and Δsag2<Δsag3, the second surface hasa convex-shaped portion in its center part and does not have aconcave-shaped portion in a periphery thereof so that a slope (Δsag),from a lens center h0 that matches the optical axis to an outerperipheral end, is first gradual, then abrupt, and gradual again beforereaching the outer peripheral end, and when a single lens centerthickness is d and a focal length of an objective lens is f, 0.9≦d/f≦1.2is satisfied.
 2. An optical pickup lens with a numerical aperture NA of0.84 or larger and an effective diameter D of 1.8≦D≦2.45 mm for focusinga light beam from a laser light source with a wavelength of 410 nm orshorter on an optical disc, the optical disc including an optical discsubstrate and a light transmitting layer in the optical disc, whereinthe optical pickup lens is a single lens with a first surface having acontinuous shape on which the light beam from the laser light source isincident and a second surface opposite to the first surface, the secondsurface facing the optical disc, the second surface has a continuousshape, a shape of the second surface of the optical pickup lens havingradii h1, h2 and h3 from an optical axis to a lens periphery, the radiisatisfying a first expression h1<h2<h3, wherein sags in the radii h1,h2, and h3 are sag1, sag2, and sag3, and differentials in the sags areΔsag1, Δsag2, and Δsag3, respectively, the differentials satisfyingsecond and third expressions 0>Δsag1>Δsag2 and Δsag2<Δsag3, the secondsurface has a convex shape in its center and does not have aconcave-shaped portion in its periphery, and for a parallel light beamfrom the laser light source, a light is converged at a point having aninter-surface distance of 0.0875 mm from a surface of the lighttransmitting layer in the optical disc.
 3. The optical pick up lensaccording to claim 2, wherein the effective diameter D is 2.04≦D≦2.45mm.
 4. The optical pick up lens according to claim 2, wherein acurvature radius of the first surface is 0.7512375≦r1≦0.9056466 mm. 5.The optical pick up lens according to claim 3, wherein a curvatureradius of the first surface is 0.7512375≦r1≦0.9056466 mm.
 6. The opticalpick up lens according to claim 2, wherein a light beam from the laserlight source enters from the first surface as a collimated light or aweak finite light in a coherent wave-front state.
 7. The optical pick uplens according to claim 3, wherein a light beam from the laser lightsource enters from the first surface as a collimated light or a weakfinite light in a coherent wave-front state.
 8. The optical pick up lensaccording to claim 4, wherein a light beam from the laser light sourceenters from the first surface as a collimated light or a weak finitelight in a coherent wave-front state.
 9. The optical pick up lensaccording to claim 5, wherein a light beam from the laser light sourceenters from the first surface as a collimated light or a weak finitelight in a coherent wave-front state.
 10. The optical pick up lensaccording to claim 2, wherein a light is converged at a point having aninter-surface distance of 0.0875 mm from a surface of the lighttransmitting layer in the optical disc, as off-axis characteristics,with an aberration of no more than 15 mλrms when the angle of view is0.3 degrees.
 11. The optical pick up lens according to claim 3, whereina light is converged at a point having an inter-surface distance of0.0875 mm from a surface of the light transmitting layer in the opticaldisc, as off-axis characteristics, with an aberration of no more than 15mλrms when the angle of view is 0.3 degrees.
 12. The optical pick uplens according to claim 4, wherein a light is converged at a pointhaving an inter-surface distance of 0.0875 mm from a surface of thelight transmitting layer in the optical disc, as off-axischaracteristics, with an aberration of no more than 15 mλrms when theangle of view is 0.3 degrees.
 13. The optical pick up lens according toclaim 5, wherein a light is converged at a point having an inter-surfacedistance of 0.0875 mm from a surface of the light transmitting layer inthe optical disc, as off-axis characteristics, with an aberration of nomore than 15 mλrms when the angle of view is 0.3 degrees.
 14. Theoptical pick up lens according to claim 6, wherein a light is convergedat a point having an inter-surface distance of 0.0875 mm from a surfaceof the light transmitting layer in the optical disc, as off-axischaracteristics, with an aberration of no more than 15 mλrms when theangle of view is 0.3 degrees.
 15. The optical pick up lens according toclaim 7, wherein a light is converged at a point having an inter-surfacedistance of 0.0875 mm from a surface of the light transmitting layer inthe optical disc, as off-axis characteristics, with an aberration of nomore than 15 mλrms when the angle of view is 0.3 degrees.
 16. Theoptical pick up lens according to claim 8, wherein a light is convergedat a point having an inter-surface distance of 0.0875 mm from a surfaceof the light transmitting layer in the optical disc, as off-axischaracteristics, with an aberration of no more than 15 mλrms when theangle of view is 0.3 degrees.
 17. The optical pick up lens according toclaim 9, wherein a light is converged at a point having an inter-surfacedistance of 0.0875 mm from a surface of the light transmitting layer inthe optical disc, as off-axis characteristics, with an aberration of nomore than 15 mλrms when the angle of view is 0.3 degrees.